Spreading resistance effects in tunneling spectroscopy of $\alpha \text{−}{\mathrm{RuCl}}_3$ and ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$

The Mott insulating state is the progenitor of many interesting quantum phases of matter including the famous high-temperature superconductors and quantum spin liquids. A recent candidate for novel spin liquid phenomena is $\alpha \text{−}{\mathrm{RuCl}}_3$, a layered honeycomb Mott insulator whose electronic structure has been a source of mystery. In particular, scanning tunneling spectroscopy has indicated a Mott gap in $\alpha \text{−}{\mathrm{RuCl}}_3$ that is much lower than the 2-eV value observed in photoemission measurements. Here, we show that the origin of this discrepancy is a spreading resistance artifact associated with tunneling into highly resistive materials by comparing with prior experiments and numerical modeling. A similar phenomenon is also observed in a substitutional alloy, ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$, that has a higher resistivity than the parent compound. While the tunneling measurements cannot be used to accurately measure the sample density of states for these materials, we can take advantage of the spreading resistance sensitivity to quantify the anisotropic resistivity of these layered materials and connect to previous macroscopic transport observations.

Figure 1

(a) Atomically resolved STM image of a cleaved $\alpha \text{−}{\mathrm{RuCl}}_3$ surface (0.6-V bias, 60 pA); (b) UPS spectra of $\alpha \text{−}{\mathrm{RuCl}}_3$ showing the occupied states.

Figure 2

(a) Tunneling conductance averaged over a 50-nm × 50-nm region at different setpoint currents; (b) Normalized tunneling conductance over a larger range of initial tunneling setpoints than in part (a) that shows the collapse of the apparent Mott gap as the tip approaches the surface. From bottom to top the spectra correspond to 0.1-, 0.2-, 0.3-, 0.4-, 0.5-, 0.6-, and 0.7-nA setpoint currents.

Figure 3

(a) Averaged normalized $dI/dV$ spectra of ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$ showing an even stronger dependence on tunneling setpoint current than for ${\mathrm{RuCl}}_3$. (b) UPS spectrum of ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$ indicating the presence of a significant gap.

Figure 4

(a) Averaged I-V spectra of $\alpha \text{−}{\mathrm{RuCl}}_3$ at different tunneling setpoint currents corresponding to Fig. 1 showing a linearization at high setpoint. (b) Averaged I-V spectra corresponding to Fig. 2 of ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$ showing an even stronger dependence on tunneling setpoint current.

Figure 5

(a) Bulk sample resistance as a function of temperature for pure ${\mathrm{RuCl}}_3$ and ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$. (b) Tip-sample resistance as a function of tunneling setpoint current extracted from Figs. 4 and 4, showing a decrease in the total resistance of the tip-vacuum-sample system. An exponential fit is fit to the trend to extract a value for the spreading resistance.

Figure 6

(a) Differential conductance for different tip-sample distances showing how distortion of a peak structure is increased as the distance decreases when spreading resistance is included; (b) Extremely small tip-sample distances that show tunneling $I\left(V\right)$ curves where essentially no remnant of the sample DOS can be observed but where the slope tends to approach the spreading resistance value $R_s$.